1. Field of the Invention
The present invention relates to a novel organic titanium compound that is useful as a titanium source in the production of a metal oxide thin film containing titanium by a metal-organic chemical vapor deposition (MOCVD) process. The present invention also relates to a stock solution containing the compound for a MOCVD process, and a method for forming a titanate dielectric thin film using the stock solution. The metal oxide thin film containing titanium is useful as not only a dielectric thin film but also as a semiconductor thin film, an optical thin film, a surface reinforcing film, and a thin film catalyst.
2. Discussion of the Background
The rapid increase in integration density of DRAMs has required that compound oxide dielectric materials be used as capacitors having higher dielectric constants than those of conventional SiO.sub.2 dielectric thin films, which have been used with great difficulty. Examples of compound oxide dielectric materials include titanium-containing compound oxides, such as, lead titanate (PT), lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT), strontium titanate (ST), barium titanate (BT), and barium strontium titanate (BST). Among them. BST is most preferable in view of dielectric characteristics.
Sol-gel processes have been vigorously studied for the production of compound oxide dielectric thin films, in which metal alkoxide solutions are applied onto substrates by spin coating methods. In the sol-gel processes, applied metal components are completely used as films without evaporation; hence the compositions of the films can be easily controlled. Capacitor electrodes for DRAMs, however, have larger steps that have complicated configurations with integration density. Thus, a uniform dielectric thin film is barely formed on an electrode substrate by a spin coat method.
For several years recently, deposition of dielectric thin-films by MOCVD processes has been actively studied in anticipation of a trend towards higher integration density of semiconductor devices, since the methods have superior step covering characteristics, that is, coating ability on complicated surfaces having steps. Organometallic compounds as raw materials generally used are organometallic complexes and metal alkoxides having .beta.-diketone ligands such as dipivaloylmethane (DPM). Alkoxide or .beta.-diketone complexes are used as sources of metals, such as Ti, Zr, and Ta, and .beta.-diketone complexes are also used as sources of Sr and Ba.
In the MOCVD process, a metal source is evaporated by heat under a reduced pressure, and then the vapor is transferred into a deposition chamber and decomposed on a substrate to deposit the resulting metal oxide on the substrate. In the above-mentioned compound oxide thin film, at least two types of organometallic compounds must be used. Since these compounds have different vaporization characteristics in the MOCVD process, control of volumes of the compounds supplied to the deposition chamber is significantly important for the control of the film composition.
At the beginning of the formation of a dielectric thin film by a MOCVD process, organometallic compounds have been directly evaporated by heat and the formed vapor has been transferred into a deposition chamber. The organometallic compounds and particularly DPM complexes that have been recommended in the MOCVD process are unstable and barely vaporizable. As a result, vaporization will be inactivated in the operation or pyrolysis will occur prior to the vapor of the compounds reaching the deposition chamber. Thus, it is difficult to achieve stable transfer of the vapor of the compounds into the deposition chamber. Expensive raw materials should be disposed after one film deposition cycle. Furthermore, the film composition is controlled with difficulty, resulting in unstable supply of thin films having superior dielectric characteristics.
In order to solve such problems, a solution feeding method has been developed, in which a stock solution of organometallic compounds dissolved in an organic solvent is supplied into a vaporization chamber placed in front of a deposition chamber, and then the vapor in the vaporization chamber is fed to the deposition chamber for film deposition. Since DPM complexes are stable in solution, the stock solution can be repeatedly used. Furthermore, the heating temperature decreases due to vaporization to prevent pyrolysis of the compounds before they reach the deposition chamber. As a result, the film composition can be readily controlled.
Japanese Patent Laid-Open No. 5-271253 discloses bis(dipivaloylmethanato)-dialkoxy titanium complex [Ti(DPM).sub.2 (OR).sub.2 wherein R is lower alkyl] as an organic titanium compound which is used as a raw material in a MOCVD process. Only bis(dipivaloylmethanato)-diisopropoxy titanium [Ti(DPM).sub.2 (O-i-Pr).sub.2 ] is disclosed as a typical example when the R is isopropyl. Japanese Patent Laid-Open No. 940683 discloses bis(dipivaloylmethanato)-di-tert-butoxy titanium [Ti(DPM).sub.2 (O-t-Bu).sub.2 ] as an organic titanium compound suitable for a solution feeding method.
In the solution feeding method, however, a low concentration solution must be used when the solubility of the organometallic compounds is low in the organic solvent; hence the deposition rate significantly decreases, resulting in inefficient film deposition. On the other hand, the use of a solution of a nearly saturated concentration causes precipitation of the compounds during the feeding of the stock solution due to evaporation of the solvent. As a result, the concentration of the stock solution varies, and the composition of the resulting film also varies. Accordingly, it is difficult to control the film composition. Some organometallic compounds are reactive with other organometallic compounds and/or the solvent, resulting in a decrease in vaporization of the compounds. As a result, it is difficult to control the film composition, and clogging will frequently occur due to the residue in the vaporization chamber and the coagulation in pipes and nozzles.
Requirements for the organometallic compounds are high solubility in an organic solvent, stable vaporization, and inertness to the other raw compounds and the solvent before the vaporization. Other important factors for the MOCVD process include high vaporization characteristics at a low temperature and high step covering characteristics over a wide temperature range from a low temperature to a high temperature.
In the formation of BST thin films, DPM complexes, i.e., Ba(DPM).sub.2 and Sr(DPM).sub.2 and their adducts are exclusively used as barium and strontium sources for the reason that any other suitable compounds are not found. As titanium sources, titanium alkoxide and titanium DPM complex are conventionally used, but these have insufficient film deposition characteristics. In the case of the formation of a BST film by a MOCVD process, titanium functions as nuclei of the film deposition. Thus, the deposition of barium and strontium is decelerated when the deposition of titanium is delayed; hence the titanium compound has significant effects on the deposition rate of the BST film.
For example, titanium alkoxides such as titanium isopropoxide among conventional titanium sources are reactive with Ba(DPM).sub.2 and Sr(DPM).sub.2, hence their vaporization characteristics significantly deteriorate. DPM complex of titanium, i.e., TiO(DPM).sub.2 is generally present as a dimer or a trimer, and it has inferior vaporization characteristics.
Use of titanium alkoxide DPM complexes is also known. Among such types of complexes which have been proposed, a complex having isopropyl groups as alkoxy groups, [Ti(DPM).sub.2 (O-i-Pr).sub.2 ], has a low vaporization temperature; however, it can be easily polymerized, resulting in deterioration of evaporation characteristics and thus a significant decrease in the film deposition rate. Another complex having n-butoxy groups as alkoxy groups, [Ti(DPM).sub.2 (O-n-Bu).sub.2 ] also has the same problem. Although a complex having t-butoxy groups as alkoxy groups. [Ti(DPM).sub.2 (O-t-Bu).sub.2 ], shows low reactivity with Ba(DPM).sub.2 and Sr(DPM).sub.2 in the solution, it has low solubility in organic solvents. Thus, the film deposition rate of this compound is also low.